The Human Heat Limit: Metabolic Rates Accelerate between 104 and 122°F. Prof. Lewis Halsey and his colleagues at the University of Roehampton in the United Kingdom have determined that the upper critical temperature (UCT) for humans is likely between 104°F and 122°F (40°C and 50°C).
Currently, additional research is being conducted to explain this increase in metabolic energy costs at elevated temperatures.
Prof. Halsey and his colleagues discovered that when people are exposed to hot and humid conditions, their resting metabolic rate, a measure of how much energy the body needs to function normally, can be elevated.
“Quite a bit of research has been conducted on the range of temperatures at which different animal species prefer to live in terms of their metabolic rates being minimal and thus their energy expenditure being low,” says Professor Halsey. “However, information regarding the upper limits of the thermal neutral zone for humans is oddly scarce.”
Understanding the temperatures at which human metabolic rates begin to increase and how this varies among individuals has implications for working conditions, sports, medicine, and international travel.
Prof. Halsey adds, “This research provides fundamental knowledge about how we respond to suboptimal environments and how ‘optimal’ varies among individuals with different characteristics.”
Prof. Halsey and his team are also examining how heart function is affected by temperatures above the UCT, as well as how the effects on the heart vary between individuals with various characteristics, such as age and physical fitness.
Prof. Halsey adds, “We found significant differences in heart function responses to heat between demographic groups, with the most striking difference being between the sexes.” Men and women’s cardiovascular responses to temperatures differ significantly, on average.
Using a cutting-edge echocardiograph, Prof. Halsey’s team measured in-depth heart function. “It was difficult to operate this equipment in the heat!” he continues. This type of apparatus is typically found in hospitals but not in research laboratories.
Professor Halsey’s presentation at the SEB conference will emphasise their most recent findings from these ongoing experiments.
“We are methodically piecing together a picture of how the body reacts to heat stress, how adaptable it is, the limits of those adaptations, and – most importantly – how variable responses are between individuals. This knowledge becomes more valuable as the world warms,” he adds.